Natural gas is the most important fuel gas in the United States and provides more than one-fifth of all the primary energy used in the United States. Natural gas is also used extensively as a basic raw material in the petrochemical and other chemical process industries. The composition of natural gas varies widely from field to field. For example, a raw gas stream may contain as much as 95% methane, with only minor amounts of other hydrocarbons, nitrogen, carbon dioxide, hydrogen sulfide or water vapor. On the other hand, streams that contain relatively large proportions of heavier hydrocarbons and/or other contaminants are common. Before the raw gas can be sent to the supply pipeline, it must usually be treated to remove at least one of these contaminants.
As it travels from the wellhead to the processing plant and ultimately to the supply pipeline, gas may pass through compressors or other field equipment. These units require power, and it is desirable to run them using gas engines fired by natural gas from the field. Since the gas has not yet been brought to specification, however, this practice may expose the engine to fuel that is of overly high Btu value, low octane number, or corrosive.
A related operation is to use field gas as combustion fuel for turbines, which are then used to drive other equipment, for example, electric power generators and compressors. In this case, the water and hydrocarbon dewpoints of the gas should be below the lowest temperature likely to be encountered en route to the turbine. If this is not done, the feed stream may contain entrained liquid water and hydrocarbons. These do not burn completely when introduced into the turbine firing chamber, and can lead to nozzle flow distribution problems, collection of liquid pools and other reliability issues. Additionally high concentrations of heavy hydrocarbons tend to make the fuel burn poorly, resulting in coke formation and deposition of carbon in the fuel pathways and on the turbine blades. These deposits reduce turbine performance and affect reliability.
There is a need, therefore, for a process that can be used in the field to lower to an appropriate level the dewpoint of gas destined for turbine fuel. The process should employ simple, robust equipment that can operate under field conditions without the need for sophisticated controls and frequent operator attendance or maintenance. The gas thus treated could then be used more reliably as turbine fuel.
That membranes can separate C.sub.3+ hydrocarbons from gas mixtures, such as natural gas, is known, for example from U.S. Pat. Nos. 4,857,078,5,281,255 and 5,501,722. Separation of acid gases from other gases is taught, for example, in U.S. Pat. No. 4,963,165. It has also been recognized that compression/condensation and membrane separation may be combined, as is shown in U.S. Pat. Nos. 5,089,033; 5,199,962; 5,205,843 and 5,374,300.
The problem of upgrading raw gas in the field, such as to sweeten sour gas, is addressed specifically in U.S. Pat. No. 4,370,150, to Fenstermaker. In this patent, Fenstermaker teaches a process that uses a membrane, selective for hydrogen sulfide and/or heavier hydrocarbons over methane, to treat a side stream of raw gas. The process produces a membrane residue stream of quality appropriate for engine fuel. The contaminants pass preferentially through the membrane to form a low-pressure permeate stream, which is returned to the main gas line upstream of the field compressor.
U.S. Pat. No. 6,053,965, relates to the use of a separation membrane in conjunction with cooling to achieve upgrading of raw natural gas to run field engines.
U.S. Pat. No. 6,035,641, relates to the use of a membrane to upgrade gas containing large amounts of nitrogen, followed by use of that gas as combustion fuel for a turbine that generates electric power.